Distinguishing premature contractions in a medical device

ABSTRACT

A method and device of distinguishing premature contractions that includes a sensor sensing cardiac signals, and a processor configured to determine a plurality of intervals in response to the sensed cardiac signal, detect a premature contraction being associated with a first R-wave of the plurality of R-waves, determine whether a metric of the first R-wave is greater than a premature contraction threshold, and identify the detected premature contraction as one of a premature atrial contraction or a premature ventricular contraction in response to determining whether a metric of the first R-wave is greater than a premature contraction threshold.

CROSS-REFERENCE TO PRIORITY APPLICATION

The present application claims priority and other benefits from U.S.Provisional Patent Application Ser. No. 61/480,044, filed Apr. 28, 2011,entitled “DISTINGUISHING PREMATURE CONTRACTIONS IN A MEDICAL DEVICE”(Attorney Docket P0041925.00), incorporated herein by reference in itsentirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is hereby made to the commonly-assigned related U.S.application Ser. No. ______ (Attorney Docket Number P0041925.USU2),entitled “DISTINGUISHING PREMATURE CONTRACTIONS IN A MEDICAL DEVICE”, toSchneider et al., and application Ser. No. ______ (Attorney DocketNumber P0041925.USU3), entitled “DISTINGUISHING PREMATURE CONTRACTIONSIN A MEDICAL DEVICE”, to Schneider et al. filed concurrently herewithand incorporated herein by reference in it's entirety.

TECHNICAL FIELD

The present disclosure relates generally to cardiac monitoring systemsand, in particular, to a method and apparatus for distinguishingpremature contractions in a medical device.

BACKGROUND

Patients suffering from heart failure can experience severe symptomsleading to hospitalization as their heart failure worsens. It isdesirable to prevent hospitalization and worsening heart failuresymptoms by managing medications and other heart failure therapies, suchas cardiac resynchronization therapy (CRT). However, clinicians arechallenged in detecting a worsening state of heart failure patientbefore the patient becomes overtly symptomatic and hospitalization isrequired. A need remains for medical devices and methods for ambulatorymonitoring of heart failure patients that improves early detection of aworsening heart failure condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an implantable medical device (IMD)optionally coupled to a lead positioned within a heart in a patient'sbody.

FIG. 2 is a functional block diagram of one embodiment of the IMD shownin FIG. 1.

FIG. 3 is a flow chart of a method for monitoring HRT in a patient.

FIG. 4 is a flow chart of a method for monitoring HRT according to analternative embodiment.

FIG. 5A is a representative R-wave showing amplitude metrics that may bedetermined for use in confirming a premature ventricular contraction(PVC).

FIG. 5B is a representative R-wave showing area metrics that may bedetermined for use in confirming a PVC.

FIG. 6 is a flow chart of a method for establishing morphology-relatedmetrics and thresholds for positively identifying PVCs during HRTmonitoring.

FIG. 7 is a flowchart of a method of distinguishing prematurecontractions as being one of a premature atrial contraction and apremature ventricular contraction, according to an embodiment of thedisclosure.

FIG. 8 is a flowchart of a method of distinguishing prematurecontractions as being one of a premature atrial contraction and apremature ventricular contraction, according to an embodiment of thedisclosure.

FIG. 9 is a flowchart of a method of distinguishing prematurecontractions as being one of a premature atrial contraction and apremature ventricular contraction, according to an embodiment of thedisclosure.

DETAILED DESCRIPTION

In the following description, references are made to illustrativeembodiments. It is understood that other embodiments may be utilizedwithout departing from the scope of the disclosure. As used herein, theterm “module” refers to an application specific integrated circuit(ASIC), an electronic circuit, a processor (shared, dedicated, or group)and memory that execute one or more software or firmware programs, acombinational logic circuit, or other suitable components that providethe described functionality.

An impaired autonomic nervous system (ANS) is associated with highermortality and hospitalization risk in patients. Heart rate turbulence(HRT) is a physiological response of the sinus node to a prematureventricular contraction (PVC). HRT can be used as a measure of thehealth of the ANS. A PVC causes a brief disturbance of heart rate andarterial blood pressure. A PVC is a ventricular depolarization, alsoreferred to as a ventricular ectopic beat, arising from the ventricularmyocardium, rather than arising from the sinoatrial node and beingconducted normally from the atria to the ventricles through the heart'snatural conduction system.

When a PVC occurs, the heart typically does not have ample time to fillwith blood prior to the premature beat, thus resulting in reducedarterial blood pressure and blood flow. In a healthy person, this changein blood pressure typically stimulates baroreceptors, which are nerveendings in the vasculature that are sensitive to changes in bloodpressure. When the baroreceptors are stimulated, a neural reflex affectsthe heart and vasculature to increase heart rate and blood pressure inan attempt to restore the body to its normal state. Until the normalpressure can be restored, however, oscillations in the patient's heartrate is frequently observed due to the changes in cardiac outputoriginating with the PVC beat. If the patient is in good health, theresponse to changes in cardiac output and arterial pressure isrelatively large and the heart recovers relatively quickly. If thepatient has an impaired ANS diminishing the arterial baroreflex, heartrate oscillations may be depressed. Accordingly, the duration andmagnitude of heart rate turbulence (HRT) following a heart beatperturbation can be a good indicator of the health of the ANS of thepatient. In particular, measuring HRT following a PVC is believed to beuseful in identifying patients at risk for congestive heart failure(CHF), CHF decompensation, sudden cardiac death, and other forms ofheart disease.

A PVC can be observed as a short ventricular cycle, i.e. the intervalbetween two consecutive R-waves, with no intervening atrial beat. ForHRT calculation it is important that only PVCs are used and notpremature atrial contractions (PACs) because the response of the sinusrhythm after a PAC produces different results. The different responsefollowing a PAC, if measured and included with measurements associatedwith a true PVC, would confound HRT measurements. A PAC can be conductedto the ventricles and appear like a short ventricular cycle, potentiallybeing detected as a PVC when PVC detection is based on intervalsmeasured between R-waves. In a dual chamber device having both atrialand ventricular sensing electrodes or when using multi-lead ECG signals,PVCs can be readily identified by a shortened ventricular cycle lengthwithout an intervening atrial depolarization signal, i.e. a P-wave.

In a cardiac monitoring device that relies on a single ECG lead,subcutaneous electrodes, or a single chamber device having electrodeslocated only in a ventricular chamber, it can be difficult todifferentiate PACs and PVCs based on the absence of a P-wave during ashort ventricular cycle because the P-wave signal is of very lowamplitude or absent. Furthermore, the available processing power in asmall implantable device, such as an implantable ECG recorder orhemodynamic monitor, may be limited precluding highly complex signalanalysis methods. Automatic and reliable identification of PVCsoriginating in the ventricles, i.e. ventricular ectopic beats, for HRTassessment that requires only a single ECG lead or EGM signal withoutrequiring high processing burden is needed.

In addition to subcutaneous or external cardiac monitors, therelationship between HRT and cardiac health can be beneficiallyexploited in other implantable medical devices (IMDs) such aspacemakers, implantable cardioverter-defibrillators (ICDs), an automaticexternal defibrillator (AED) or heart monitor and the like. According tovarious embodiments, an implantable medical device (IMD) detects PVCsand monitors HRT resulting from a PVC to determine an indication of thepatient's cardiac health. The perturbation may be naturally occurring inthe patient. HRT measurements made by the IMD can be used for enhancedmonitoring, diagnosis and/or therapeutic functions in response to themeasured turbulence. For example, the IMD may store diagnostic data in amemory, activate an alarm to the patient if medical attention ispotentially warranted, or the like. In further embodiments, the IMDadministers or adjusts an appropriate therapy or other response whensuch treatment or adjustment to the treatment is needed. As used herein,the term “response” is intended to broadly encompass any type of medicalresponse, alarm, report, telemetered data or the like (including storageof data within the IMD), as well as any of the various therapies thatmay be provided by the IMD to the patient. In a further embodiment, HRTmay be used to determine optimal settings for a pacemaker, or foroptimal delivery of a pharmaceutical or other therapy.

FIG. 1 is a schematic diagram of an IMD 10 optionally coupled to a lead14 positioned within a heart 8 in a patient's body 6. IMD 10 maycorrespond to a variety of implantable medical devices including an ECGmonitor, cardiac pacemaker, implantable cardioverter defibrillator(ICD), implantable hemodynamic monitor, a drug pump, a neurostimulatoror the like. IMD 10 may or may not be provided with therapy deliverycapabilities. When provided with therapy delivery capabilities, IMD 10may be coupled to additional leads and/or catheters operativelypositioned relative to the patient's heart 8 or other body tissues fordeploying stimulating/sensing electrodes, physiological sensors, and/ordrug delivery ports. While lead 14 is shown carrying sense/paceelectrodes 16 and 18 positioned within the right ventricle of thepatient's heart in the illustrative embodiment, it is recognized thatlead 14 may be configured to extend transvenously into other heartchambers or blood vessels or subcutaneously away from IMD 10 to otherbody locations for positioning any number of electrodes and/orphysiological sensors in a desired location.

In one embodiment, IMD 10 corresponds to an implantable cardiac signalmonitor capable of at least sensing an ECG or intracardiac EGM signalusing an intracardiac lead 14 and/or subcutaneous electrodes 42 and 44incorporated in the housing 12 of IMD 10. Subcutaneous electrodes mayadditionally or alternatively be carried by a lead. IMD 10 receives theECG and/or EGM signals, collectively referred to herein as “cardiacelectrical signals” or simply “cardiac signals”.

Housing 12 encloses circuitry (not shown in FIG. 1) included in IMD 10for controlling and performing device functions and processing sensedsignals as described herein. Cardiac signals may be stored and/oranalyzed by IMD 10 for diagnostic or prognostic purposes. Cardiacarrhythmias, heart rate, premature contractions, and other events may bedetected and corresponding data may be stored by IMD 10.

In particular and as further described herein, IMD 10 will detect PVCsand evaluate cardiac signals subsequent to the disturbance associatedwith a PVC for computing metrics of HRT. HRT metrics are then availableto a clinician for diagnosis and prognosis of various heart-relatedconditions.

IMD 10 is capable of bidirectional communication with an external device26 via bidirectional telemetry link 28. Device 26 may be embodied as aprogrammer, typically located in a hospital or clinic, used to programthe operating mode and various operational variables of IMD 10 andinterrogate IMD 10 to retrieve data acquired and stored by IMD 10.Device 26 may alternatively be embodied as a home monitor used forremote patient monitoring for retrieving data from the IMD 10. Datastored and retrieved from IMD 10 may include data related to IMDfunction determined through automated self-diagnostic tests as well asphysiological data acquired by IMD 10 including HRT data.

External device 26 is further shown in communication with a centraldatabase 24 via communication link 30, which may be a wireless orhardwired link. Programming data and interrogation data may betransmitted via link 30. Central database 24 may be a centralizedcomputer, Internet-based or other networked database used by a clinicianfor remote monitoring and management of patient 6. An example of aremote patient management system in which tissue oxygenation monitoringmay be incorporated for monitoring heart failure patients is generallydescribed in commonly-assigned U.S. Pat. No. 6,599,250 (Webb, et al.),hereby incorporated herein by reference in its entirety. It isrecognized that other external devices, such as other physiologicalmonitoring devices or other types of programming devices, may be used inconjunction with IMD 10 and incorporate portions of the methodsdescribed herein.

The methods described herein for analyzing cardiac signals to determineHRT metrics may be implemented in IMD 10 and the results of dataanalysis transmitted to external device 26 upon an interrogation requestand optionally on to central database 24. IMD receives a cardiac signal,detects PVCs, computes HRT metrics and stores the metrics fortransmission to external device 26. Alternatively, raw cardiac signaldata may be transmitted from IMD 10 to external device 26 with the dataanalysis performed by external device 26 and/or central database 24. Instill other embodiments, the signal analysis may be performed in adistributed manner across the IMD 10, external device 26, and/or centraldatabase 24. For example, IMD 10 may detect PVCs and transmit onlycardiac signal segments containing a valid PVC for HRT analysis byexternal device 26 or central database 24.

FIG. 2 is a functional block diagram of one embodiment of IMD 10. IMD 10generally includes timing and control circuitry 52 and an operatingsystem that may employ microprocessor 54 or a digital state machine fortiming sensing and therapy delivery functions (when present) inaccordance with a programmed operating mode. Microprocessor 54 andassociated memory 56 are coupled to the various components of IMD 10 viaa data/address bus 55.

IMD 10 may include therapy delivery module 50 for delivering a therapyunder the control of microprocessor 54 in response to determining a needfor therapy, e.g., based on sensed physiological signals. Therapydelivery module 50 may provide drug delivery therapies or electricalstimulation therapies, including neurostimulation or cardiac pacing,cardiac resynchronization therapy, or anti-arrhythmia therapies.Therapies are delivered by module 50 under the control of timing andcontrol circuitry 52.

Therapy delivery module 50 may be coupled to two or more electrodes 68via an optional switch matrix 58 for delivering an electricalstimulation therapy such as cardiac pacing or neurostimulation.Electrodes 68 may correspond to any of the electrodes 16, 18, 42 and 44shown in FIG. 1.

Electrodes 68 are also used for receiving cardiac electrical signalsthrough any unipolar or bipolar sensing configuration. Cardiacelectrical signals are monitored for diagnostic or prognostic purposes,managing a patient condition, and may be used for determining when anautomatically-delivered therapy is needed and controlling the timing anddelivery of the therapy. Signal processor 60 receives cardiac signalsand includes sense amplifiers and may include other signal conditioningcircuitry and an analog-to-digital converter. Cardiac electrical signalsreceived from electrodes 68, which may be intracardiac EGM signals, farfield EGM signals, or subcutaneous ECG signals, are used for detectingPVCs and determining HRT metrics.

IMD 10 may be coupled to one or more sensors 70 of physiological signalsother than cardiac electrical signals. Physiological sensors may includea pressure sensor, activity sensor, oxygen sensor or the like. Sensorsignals are received by sensor interface 62, which may provide initialamplification, filtering, rectification, or other signal conditioning.Sensor signals are used by signal processor 60 and/or microprocessor 54for detecting physiological events or conditions.

The operating system includes associated memory 56 for storing operatingalgorithms and control parameter values that are used by microprocessor54. The memory 56 may also be used for storing data compiled from sensedcardiac signals and/or relating to device operating history fortelemetry out on receipt of a retrieval or interrogation instruction.Microprocessor 54 may respond to cardiac signals by altering a therapy,triggering data analysis and storage, or triggering alert 74 to generatean alert signal to the patient or a clinician that a condition has beendetected that may require medical intervention. Data relating tophysiological signal processing may be stored in memory 56 for laterretrieval. For example, PVC detection may trigger a HRT analysis anddata storage. When PVC detections, a HRT metric or trend thereof reachesa predetermined threshold, an alert signal may be generated by alertmodule 74 in the form of an audible sound, vibration, transmittablemessage or other notification.

FIG. 3 is a flow chart 100 of a method for monitoring HRT in a patient.Flow chart 100 and other flow charts presented herein are intended toillustrate the functional operation of the device, and should not beconstrued as reflective of a specific form of software or hardwarenecessary to practice the methods described. It is believed that theparticular form of software will be determined primarily by theparticular system architecture employed in the device and by theparticular detection and therapy delivery methodologies employed by thedevice. Providing software to accomplish the described functionality inthe context of any modern IMD, given the disclosure herein, is withinthe abilities of one of skill in the art.

Methods described in conjunction with flow charts presented herein maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

At block 102, cardiac signal sensing is performed using one or moreselected electrode pairs. At block 104, R-waves are sensed from thecardiac signal. R-wave sensing may be performed according to any methodimplemented in the IMD. Typically, an automatically adjusting thresholdis applied to the cardiac signal for sensing R-waves as generallydescribed in U.S. Pat. No. 5,117,824 (Keimel, et al.), herebyincorporated herein by reference in its entirety. At block 105,intervals between consecutively sensed R-waves, i.e. RR intervals, areanalyzed to determine if an RR interval corresponding to a PVC couplinginterval is detected as determined at decision block 106. Criteria fordetecting a PVC coupling interval may vary between embodiments. Onemethod for detecting a PVC coupling interval is described in greaterdetail in conjunction with FIG. 4. Generally, a PVC coupling interval isdetected as an interval that is shorter than at least one or morepreceding RR intervals. Additionally or alternatively, one or moresubsequent RR intervals may be compared to a given RR interval fordetecting a PVC coupling interval.

If a PVC coupling interval is detected, as determined at decision block106, the process advances to block 108 to identify an R-wave fiducialpoint for each of the PVC QRS complex and a selected number N precedingsensed R-waves. As long as a PVC coupling interval is not detected, theprocess continues to sense R-waves (block 104) and analyze RR intervals(block 105). The fiducial point identified at block 108 may correspondto the R-wave amplitude at the time of R-wave detection, a maximum peakR-wave amplitude (of a rectified or non-rectified signal), a minimumR-wave peak, a maximum slope, or other selected point along the QRScomplex.

At block 110, at least one time interval is established beginning orending at the fiducial point and extending a predetermined time from thefiducial point for each of the PVC and N preceding beats. The area underthe R-wave during the established time interval is computed at block 112for the PVC and the N preceding beats. The predetermined time may be inthe range of approximately 25 ms to approximately 100 ms or more an mayvary between embodiments. The time interval may generally be defined tomaintain the time interval within an expected QRS complex width.Alternatively, the time interval may be defined to extend beyond anexpected normal QRS complex width (i.e. beyond a beginning or end of anormal QRS width), to allow a wider QRS width associated with a PVC tobe detected based on a change in area under the R-wave signal during theestablished time interval.

The computed area under the R-wave during the established time intervalfor the suspected PVC detected based on the detected PVC couplinginterval is compared to the area(s) computed for the N preceding beatsat block 114. A representative area for the N preceding beats may bedetermined for comparison to the area computed for the suspected PVC.For example an average, median, nth maximum area or other statisticalmeasure may be determined for the N preceding beats. A difference orratio of the suspected PVC area may then be computed and compared to apredefined PVC detection threshold. Alternatively, a difference or ratioof the suspected PVC area and each of the N preceding beats may becomputed and the computed differences or ratios combined in a summation,average, or other representative number are compared to a PVC detectionthreshold. It is understood that many specific implementations may beconceived for detecting a clinically significant difference between anarea computed for a suspected PVC and an area computed for one or morepreceding beats.

If the PVC area is not different (negative result at decision block114), the suspected PVC is rejected at block 116. In other words thesuspected PVC based on a detected PVC coupling interval is rejected asnot being a PVC. The PVC coupling interval detected at block 106 may beassociated with a PAC, oversensing of a T-wave or other non-cardiacrelated signal noise. If the PVC detection is rejected at block 116, theprocess returns to block 104 to continue sensing R-waves and analyzingRR intervals for detecting a PVC coupling interval. HRT metrics are notcomputed.

If the PVC detection threshold criteria applied to the computed areas issatisfied at decision block 114, a PVC detection is confirmed at block118. PVC detection triggers an analysis of RR intervals preceding andsubsequent to the detected PVC and computation of HRT metrics at block120. It is noted that HRT metrics such as Turbulence Onset andTurbulence Slope are usually computed using RR intervals measured inresponse to multiple PVCs, not just one PVC detection. If a HRT metricreaches an alert threshold at block 122, an alert is generated at block124 to notify the patient and/or clinician of the detected condition andallow the clinician to assess the patient's condition and take anyappropriate intervention. If an alert threshold is not reached, patientmonitoring continues by returning to block 104.

It is recognized that in addition to or alternatively to generating analert at block 124 the IMD may be configured to provide other responsesto the HRT analysis. An IMD capable of delivering a cardiac therapy maybe triggered to adjust the therapy based on the HRT metric. Furthermore,whenever HRT metrics are computed at block 120, it is understood that ifthe metrics are being computed by a processor incorporated in the IMD,the HRT metrics are stored for transmission to an external device asdescribed in conjunction with FIG. 1. The digitized cardiac signal ordata associated with the PVC detection, preceding beats, and/orsubsequent beats may be stored along with HRT metrics for review by aclinician. In an another embodiment, confirmed detection of the PVC atblock 118 causes storage of a cardiac signal strip including precedingand subsequent beats needed for HRT analysis to be performed by anexternal device after transmission of the cardiac signal strip to theexternal device.

FIG. 4 is a flow chart 200 of a method for monitoring HRT according toan alternative embodiment. At block 202, cardiac signal sensing isinitiated. R-waves are sensed at block 204 according to an R-wavedetection method implemented in the IMD. At block 206, a reference RRinterval is computed. The reference RR interval is computed to provide abaseline RR interval used in identifying an unexpectedly short RRinterval associated with a possible PVC coupling interval. A referenceRR interval may be computed as a running average or a median of a mostrecent number of RR intervals. For example, in one embodiment, thereference RR interval is computed as the median of the most recent 11 RRintervals.

At block 208, a current RR interval is compared to a prematuritythreshold for determining whether a PVC coupling interval is detected.The prematurity threshold is defined as a percentage of the reference RRinterval. In one embodiment, a prematurity of 80%, i.e. an RR intervalthat is less than 80% of the reference RR interval, is detected aspotential PVC coupling interval.

If an RR interval less than the prematurity threshold is detected atblock 208, the immediately subsequent RR interval is compared to a pausethreshold at decision block 210. A PVC is normally followed by a long RRinterval, often referred to as the “compensatory pause”. A pausethreshold for detecting the compensatory pause following a potential PVCis defined as a percentage of the reference RR interval in oneembodiment. For example, if the RR interval immediately following apotential PVC coupling interval is greater than 120% of the reference RRinterval, a preliminary PVC detection is made at block 211.

If RR intervals do not meet the timing-related criteria for detecting aPVC applied at decision steps 208 and 210, the process continues tosense R-waves at block 204 and measure RR intervals at block 206. Ifthese timing-related criteria are satisfied at blocks 208 and 210,morphology-related metrics of the susptected PVC are determined atblocks 212 and 214. The morphology-related metrics are used to eitherconfirm or reject the preliminary PVC detection.

At block 212, amplitude metrics are determined. FIG. 5A is arepresentative R-wave 300 showing amplitude metrics that may bedetermined for use in confirming a PVC. In one embodiment, a maximumamplitude 302, a minimum amplitude 304, and a max-min difference 306 aredetermined.

At block 214, a fiducial point is identified and area metrics arecomputed at block 216 using the fiducial point. FIG. 5B is arepresentative R-wave 310 showing area metrics that may be determinedfor use in confirming a PVC. A fiducial point 312 is identified forestablishing time intervals over which an area defined between theR-wave and a baseline 314 is computed. In the example shown, the maximumR-wave peak is identified as the fiducial point 312. In an alternativeembodiment, the point at which the R-wave crosses a sensing thresholdfor R-wave detection is identified as the fiducial point. In still otherembodiments, a fiducial point may be identified as a peak slope orbaseline crossing.

After identifying the fiducial point, a time interval 316 is definedrelative to fiducial point 312. Time interval 316 ends at the time ofthe fiducial point 312 and extends a predetermined time interval, X ms,earlier than the fiducial point. A second time interval 318 is definedbeginning at the fiducial point 312 and ending a predetermined timeinterval, X ms, later than the fiducial point.

A first area 320 under the R-wave 310 is computed over the interval 316ending at the fiducial point 312. A second area 322, collectively 322Aand 322B, defined by R-wave 310 and baseline 314 is computed over timeinterval 318. In some embodiments, the absolute values of the areas 322Aand 322B are summed or R-wave 310 may be rectified such that both areas322A and 322B are positive-going areas and are summed to obtain a totalarea corresponding to time interval 318. In other embodiments, thenegative-going area 322B may be subtracted from the positive-going area322A to obtain a net positive area over the interval 318 correspondingto the difference between 322A and 322B.

In FIG. 5B, the time intervals 316 and 318 are shown to extend for equaltime intervals of X ms. In alternative embodiments, two distinct timeintervals are defined for establishing a first time interval ending atthe fiducial point 312 and a second time interval beginning at thefiducial point 312. The second time interval may extend for a differentamount of time from the fiducial point than the first time interval. Thetime intervals 316 and 318 may each extend between 0 ms and 100 ms ormore from the fiducial point, for example, approximately 25 ms,approximately 50 ms, or approximately 100 ms, or more from the fiducialpoint 312. In one embodiment, a fiducial point is identified as themaximum positive-going slope of R-wave 310. A first time interval isdefined extending approximately 50 ms earlier than the maximumpositive-going slope and a second time interval is defined extendingapproximately 100 ms. In another embodiment, the first and second areasextending 50 ms and 100 ms before and after a maximum peak of arectified R-wave are determined.

Area metrics that are computed for detecting a PVC may include a firstarea 320 defined by a time interval 316 that ends at the fiducial point312, a second area 322 that begins at fiducial point 312, a sum of areas320 and 322, a ratio of areas 320 and 322, or a difference of areas 320and 322. While two areas 320 and 322 are shown and described, it isrecognized that multiple areas computed over respective time intervalsdefined using one or more fiducial points may be used individually or invarious combinations to determine an area metric for use in detecting aPVC.

Referring again to FIG. 4, R-wave area metrics are computed at block 216using the fiducial point and defined time intervals relative to thefiducial point. At decision block 218, the area metrics and theamplitude metrics are compared to morphology criteria for detecting aPVC. A threshold for each of the metrics is predetermined to distinguishbetween a PVC and a PAC or other noise or artifact.

In one embodiment, the area metrics determined at block 216 aredetermined for both the suspected PVC beat and one or more immediatelypreceding beats. In this way, the generation and storage of a predefinedmorphology template for detecting a PVC is not needed. A ratio ordifference between the area metric for the suspected PVC beat and thecorresponding area metric for the immediately preceding beat isdetermined and compared to a PVC detection threshold at block 218.

For example, in one embodiment, the area under the R-wave over a 50 msinterval ending at the maximum peak of the R-wave is computed for thesuspected PVC and the immediately preceding beat. The difference betweenthese areas is determined and compared to a predetermined thresholdfound to distinguish PVCs and PACs with a high degree of certainty. Inanother embodiment, a sum of a first area extending for 50 ms precedingthe maximum R-wave peak and a second area extending for 100 ms followingthe maximum R-wave peak is determined as a summed area for each of thesuspected PVC beat and the immediately preceding beat. The ratio of thesummed areas is compared to a predetermined threshold that distinguishesPVCs from PACs.

The thresholds used to compare to R-wave amplitude and area metrics forconfirming a PVC detection may be established based on clinical datafrom a population of patients. Alternatively, thresholds may beindividualized for a particular patient through analysis of naturallyoccurring and/or pacing-induced PACs and PVCs.

If the morphology-related criteria are not met at decision block 218,the preliminary PVC detection is rejected and the process returns toblock 204 to continue sensing R-waves. If morphology-related criteriaare satisfied at block 218, the preliminary PVC detection is confirmedat block 220. The valid PVC detection triggers computation of HRTmetrics at block 222. HRT data is stored for later retrieval and reviewby a clinician. If a HRT metric reaches a response threshold, the IMDmay respond accordingly. For example, if an alert threshold is reachedas determined at block 224, a patient or clinician alert is generated atblock 226 or another response is provided.

It is understood that although the preliminary PVC detection isdescribed as being satisfied in Block 211 of FIG. 4 when intervalsrequirements are met, and therefore the timing-related criteria appliedat Block 208 and 210 are shown to be determined prior to themorphology-related criteria of Blocks 212-218, the order of detectionbetween the time-related and the morphology-related criteria could bereversed, so that the preliminary PVC detection could be based on themorphology-related criteria, and followed by the time-related criteriaif the morphology-related criteria are satisfied.

FIG. 6 is a flow chart 400 of a method for establishingmorphology-related metrics and thresholds for positively identifyingPVCs during HRT monitoring. The particular amplitude or area metric thatdistinguishes PACs and PVCs with the highest degree of confidence mayvary between patients due to the particular lead and electrodeconfiguration used and other factors. As such, an analysis may beperformed to compare the sensitivity, specificity and confidence ofmultiple amplitude and/or area metrics in correctly distinguishing PACsand PVCs for a given patient or patient population. The process shown inflow chart 400, therefore, may be performed using cardiac signalsacquired from an individual patient or from a population of patients.Cardiac signals may be acquired over a period of time to collect anumber of spontaneously occurring PACs and PVCs. Additionally oralternatively, cardiac signals may be acquired during an office visit inwhich PACs and PVCs are induced by pacing pulses.

The statistical computations described in the following description maybe too high of a computational burden to be practically implemented inan implantable device. In some embodiments, the process shown in flowchart 400 is implemented in an IMD system including an externalprocessor, such as the system in FIG. 1. Statistical analysis andestablishment of PVC detection criteria may be performed by the externalprocessor and then stored or programmed in the IMD, or a small wearableexternal device, for use in ambulatory patient monitoring.

At block 404, R-waves are sensed. At block 406, a number of prematurecontractions are sensed and/or induced, including both PACs and PVCs.Spontaneous premature contractions are detected from the cardiac signalbased on RR interval measurements or based on a known premature pacingpulse. As described previously, spontaneous premature beats may bedetected based on RR intervals being less than a prematurity thresholdfollowed by RR intervals being greater than a pause threshold.

For each premature beat, one or more amplitude metrics are computed atblock 408 and one or more area metrics are computed at block 410. Theamplitude metrics may include the maximum peak, the minimum peak and thepeak-to-peak amplitude as shown in FIG. 5A. Computation of the areametrics involves first identifying a fiducial point as describedpreviously then establishing time intervals beginning and/or ending atthe fiducial point and computing an area bounded by the R-wave and thebaseline over each of the established time intervals. The time intervalsmay extend, for example, 50 ms earlier, 100 ms earlier, 50 ms later and100 ms later than the fiducial point and/or other selected timeintervals. Area metrics may include an area over a single time intervalor summed areas occurring over an earlier (ending at the fiducial point)and later (beginning at the fiducial point) time interval.

At block 412 the premature contractions are distinguished as PVCs andPACs to allow computation of mean values for each amplitude and areametric separately for PACs and for PVCs. Distinguishing PVCs and PACsmay be performed manually by an expert or automatically based on anatrial signal if one is available for determining if a P-wave precedesan R-wave occurring at a premature coupling interval. When PACs and PVCsare pacing induced, the mean metrics are computed separately for PACsand PVCs according to the known premature pacing pulses.

At block 414, the mean values for each metric are used for determiningan optimal threshold for each metric that separates the PACs from thePVCs with a high degree of confidence. In one embodiment, a thresholdfor each amplitude and area metric is determined that results in themaximum Chi Square test statistic for separating PACs from PVCs for thatmetric. Each metric and corresponding optimal threshold are thenevaluated at block 416 to determine a minimized set of area and/oramplitude metrics for use in positive PVC identification. This minimizedset of discriminatory morphology metrics allows discrimination of PVCsfrom PACs (or other noise or artifact) using a ventricular orsubcutaneous cardiac signal only.

In one embodiment a performance score is computed at block 416 forscoring the performance of each metric in positively identifying PVCswith a high degree of confidence. The score may be a weightedcombination of statistical measures of the metric threshold performancein separating PVCs and PACs. For example, the Chi Square test statistic,PVC sensitivity as a percentage of all premature beats, PVC specificityas a percentage of all premature beats, the positive predictive power asa percentage, and the negative predictive power as a percentage may allbe calculated for an optimal threshold identified at block 414 for agiven metric.

For each statistical measure, the best performing thresholds areidentified and given points according to a point value systemestablished to rate the best performing metrics. For example, the bestperforming metric based on a given statistical measure is identified andgiven 3 points; the second best performing threshold based on the samestatistical measure is identified and given 2 points, and the third bestperforming threshold is identified and given 1 point. More specifically,the amplitude metric (e.g., max, min or peak-to-peak) having the highestChi Square test statistic may be given 3 points and the amplitude metrichaving the lowest Chi Square test statistic may be given 1 point.Similar point assignments are made for the amplitude metrics for each ofthe other statistical indicators, i.e. sensitivity, specificity,positive predictive power and negative predictive power. Each areametric is likewise awarded points based on the best performing to theworst performing metric according to each individual statisticalmeasure.

The received points for a given metric threshold are summed to determinean overall performance score for the corresponding metric. Theperformance score represents a combination of the individual statisticalmeasures for a given amplitude or area metric. The amplitude and areametrics may be assessed separately to identify a best performingamplitude metric and a best performing area metric. Alternatively, theamplitude and area metrics may be grouped and awarded points todetermine the overall best performing metrics.

At block 418, PVC detection criteria are established based on theperformance scores computed at block 416. In one embodiment, the bestperforming amplitude metric and the best performing area metric areidentified. The optimal thresholds identified for these best performingmetrics are stored at block 420 establishing PVC detection criteria fora given patient or population of patients. These established detectioncriteria are used during patient monitoring for detecting PVCs fortriggering a HRT monitoring algorithm.

The criteria established at block 418 may require that an amplitudemetric and an area metric for a suspected PVC each meet a respectivethreshold requirement in order to confirm a time-interval based PVCdetection. Alternatively, a best amplitude metric and a best area metricmay be combined and a combined threshold identified and stored at block420. For example, the best amplitude metric and the best area metric maybe combined in a weighted combination. A corresponding threshold appliedto the combined morphology metric would be computed or scaledappropriately for application to the weighted combination for reliablediscrimination of PVCs.

In one embodiment, amplitude and area metrics are computed as ratios ofa morphology metric measured from the suspected PVC beat and thecorresponding metric measured from the immediately preceding beat. Thebest performing amplitude metric and the best performing area metric caneasily be combined in a single metric when both metrics are determinedas ratios. In a specific example, a ratio of the maximum R-waveamplitude of a suspected PVC and the maximum R-wave amplitude of theimmediately preceding beat may be identified as a best performingamplitude metric. A ratio of a summed area for the PVC beat and thesummed area for the immediately preceding beat may be identified as thebest performing area metric, where the summed area is the combined areaunder the R-wave over a first time interval preceding the R-wave maximumpeak by 50 ms and a second time interval extending from the R-wavemaximum peak for 100 ms. In an empirical study performed by theinventors, these metrics were found to be the best performing metricswith an optimal threshold of approximately 40% for the summed area ratioand approximately 20% for the amplitude ratio. As such, a thresholdcriterion established at block 418 may require that a sum of the summedarea ratio and twice the amplitude ratio must be greater thanapproximately 80% (40% plus twice 20%) to positively detect a PVC.

In other embodiments, a single amplitude or area parameter identified ashaving the highest performance score may be identified at block 416. ThePVC detection criterion established at block 418 requires the singlemetric to meet the optimal threshold determined at block 414 forpositively detecting a PVC.

FIG. 7 is a flowchart of a method of distinguishing prematurecontractions as being one of a premature atrial contraction and apremature ventricular contraction, according to an embodiment of thedisclosure. As illustrated in FIG. 7, according to one embodiment,during distinguishing of a premature contraction as being one of a PVCand a PAC, R-waves are sensed from a sensed cardiac signal, Block 500,and intervals between consecutively sensed R-waves, i.e., RR intervals,are analyzed to determine if an RR interval corresponding to a prematurecontraction is detected, Block 502. In order to detect the occurrence ofa premature contraction, for example, the device determines whether adetected RR interval is less than a predetermined threshold. Thepremature contraction threshold may be a fixed value, or may bedetermined based on one or more preceding RR intervals and/or one ormore subsequent RR intervals.

If a premature contraction is not detected, the device continues sensingcardiac signals, Block 500. If a premature contraction is detected, ametric of the detected RR-interval is determined and compared to apremature contraction threshold. For example, a maximum amplitude of theR-wave associated with the detected RR interval corresponding to thepremature contraction is determined, Block 504, and a determination ismade as to whether the maximum amplitude of the R-wave is greater than apredetermined threshold, Block 506. According to one embodiment, forexample, the predetermined threshold corresponds to a percentageincrease of the maximum amplitude from the amplitude of one or moreRR-intervals occurring prior to the RR-interval associated with thedetecting of the premature contraction, such as 20 percent for example.

If the maximum interval is less than or equal to the predeterminedthreshold, i.e., less than or equal to a 20 percent increase from one ormore previous RR-intervals, the premature contraction is identified asbeing associated with a PAC, Block 508, and the device continues sensingcardiac signals, Block 500. If the maximum interval is determined to begreater than the predetermined threshold, i.e., greater than a 20percent increase from one or more previous RR-intervals, the prematurecontraction is identified as being associated with a PVC, Block 510.

According to an embodiment of the disclosure, if the prematurecontraction is determined to be associated with a PVC, heart rateturbulence metrics may be determined and stored, Block 512, and once analert threshold associated with the determined PVCs is reached, Yes inBlock 514, a patient or clinician alert is generated, Block 516, asdescribed above. If the alert threshold has not been reached, No inBlock 514, and the device continues sensing cardiac signals, Block 500.

FIG. 8 is a flowchart of a method of distinguishing prematurecontractions as being one of a premature atrial contraction and apremature ventricular contraction, according to an embodiment of thedisclosure. As illustrated in FIG. 8, according to one embodiment,during distinguishing of a premature contraction as being one of a PVCor a PAC, R-waves are sensed from a sensed cardiac signal, Block 600,and intervals between consecutively sensed R-waves, i.e., RR intervals,are analyzed to determine if an RR interval corresponding to a prematurecontraction is detected, Block 602. In order to detect the occurrence ofa premature contraction, for example, the device determines whether adetected RR interval is less than a predetermined threshold. Thepremature contraction threshold may be a fixed value, or may bedetermined based on one or more preceding RR intervals and/or one ormore subsequent RR intervals.

If a premature contraction is not detected, the device continues sensingcardiac signals, Block 600. If a premature contraction is detected, ametric of the detected RR-interval is determined and compared to apremature contraction threshold. For example, the device determines anR-wave fiducial point for both an R-wave of the detected RR intervalassociated with the premature contraction being determined and aselected number N of preceding sensed R-waves, or beats, Block 604. Atime interval is established beginning and/or ending at the determinedfiducial point and extending a predetermined time from the fiducialpoint both the R-wave of the detected RR interval associated with thedetermined premature contraction and the selected number N of precedingsensed R-waves, Block 606. According to one embodiment, for example, thetime interval is set as being 50 ms prior to the fiducial point and 100ms after the fiducial point.

The area under the R-wave fiducial points during the time interval forboth the R-wave of the detected RR interval associated with thepremature contraction being determined and the selected number N ofpreceding sensed R-waves is then determined, Block 608, and adetermination is made as to whether the area determined for the R-waveof the detected RR interval associated with the determined prematurecontraction is greater than a predetermined area threshold, Block 610.According to one embodiment, for example, the predetermined areathreshold corresponds to a percentage increase or decrease of the areaassociated with the R-wave of the detected RR interval associated withthe determined premature contraction from the N preceding sensedR-waves, such as 39 percent for example.

According to one embodiment, the area under the R-wave of the detectedRR interval associated with the premature interval is compared with thearea under the R-wave of the immediately preceding beat. In anotherembodiment, the areas of a predetermined number N of prior beats may becombined into a single metric, such as an average, for example, or acomparison is made between the percent increases to X out of Y beats,for example.

If the area determined for the R-wave of the detected RR intervalassociated with the determined premature contraction is less than orequal to the predetermined threshold, i.e., less than or equal to a 39percent increase or decrease from the N previous R-waves, the prematurecontraction is identified as being associated with a PAC, Block 612, andthe device continues sensing cardiac signals, Block 600. If the areadetermined for the R-wave of the detected RR interval associated withthe determined premature contraction is determined to be greater thanthe predetermined threshold, i.e., greater than a 39 percent increase ordecrease from one or more previous RR-intervals, the prematurecontraction is identified as being associated with a PVC, Block 614.

According to an embodiment of the disclosure, if the R-wave of thedetected RR interval associated with the determined prematurecontraction is determined to be associated with a PVC, heart rateturbulence metrics may be determined and stored, Block 616, and once analert threshold associated with the determined PVCs is reached, Yes inBlock 618, a patient or clinician alert is generated, Block 620, asdescribed above. If the alert threshold has not been reached, No inBlock 618, and the device continues sensing cardiac signals, Block 600.

FIG. 9 is a flowchart of a method of distinguishing prematurecontractions as being one of a premature atrial contraction and apremature ventricular contraction, according to an embodiment of thedisclosure. As illustrated in FIG. 9, according to one embodiment,during distinguishing of a premature contraction as being one of a PVCand a PAC, R-waves are sensed from a sensed cardiac signal, Block 700,and intervals between consecutively sensed R-waves, i.e., RR intervals,are analyzed to determine if an RR interval corresponding to a prematurecontraction is detected, Block 702. In order to detect the occurrence ofa premature contraction, for example, the device determines whether adetected RR interval is less than a predetermined threshold. Thepremature contraction threshold may be a fixed value, or may bedetermined based on one or more preceding RR intervals and/or one ormore subsequent RR intervals.

If a premature contraction is not detected, the device continues sensingcardiac signals, Block 700. Once a premature contraction is detected, ametric of the detected RR-interval is determined and compared to apremature contraction threshold. For example, the device determines anR-wave fiducial point for both an R-wave of the detected RR intervalassociated with the premature contraction being determined and aselected number N of preceding sensed R-waves, or beats, Block 704. Atime interval is established beginning and/or ending at the determinedfiducial point and extending a predetermined time from the fiducialpoint for each of the R-wave of the detected RR interval associated withthe determined premature contraction and the selected number N ofpreceding sensed R-waves, Block 706. According to one embodiment, forexample, the time interval is set as being 50 ms prior to the fiducialpoint and 100 ms after the fiducial point.

The area under the R-wave fiducial points is determined for both theR-wave of the detected RR interval associated with the prematurecontraction being determined and the area associated with a selectednumber N of preceding sensed R-waves is determined, Block 708. Thedetermination of the area corresponding to the selected number N ofpreceding R-waves is identified as an area associated with a normalbeat. A determination is made as to whether an increase or decrease of aratio of the area of the R-wave associated with the prematurecontraction being determined and the area associated with a normal beatis greater than a predetermined threshold, Block 710. According to oneembodiment, for example, the predetermined threshold corresponds to apercentage increase or decrease of a ratio of the area of the R-waveassociated with the premature contraction being determined and the areaassociated with a normal beat, such as 55 percent for example.

If the ratio of the area of the R-wave associated with the prematurecontraction being determined and the area associated with a normal beatis not greater than the area threshold, the premature contraction isidentified as being associated with a PAC, Block 712, and the devicecontinues sensing cardiac signals, Block 700. If the ratio of the areaof the R-wave associated with the premature contraction being determinedand the area associated with a normal beat is greater than the areathreshold, the premature contraction is identified as being associatedwith a PVC, Block 714.

According to an embodiment of the disclosure, if the prematurecontraction is identified as being associated with a PVC, heart rateturbulence metrics may be determined and stored, Block 716, adetermination is made as to whether the determined heart rate turbulencereaches an alert threshold, Block 718. If the alert threshold has notbeen reached, NO in Block 718, the device continues sensing cardiacsignals, Block 700. If the determined heart rate turbulence is greaterthan the alert threshold, Yes in Block 718, a patient or clinician alertis generated, Block 720, as described above. According to oneembodiment, the alert threshold corresponds to determining a combinationof heart rate turbulence parameters, such as turbulence onset andturbulence slope, for example, so that the alert threshold is adetermination of whether none, one or both of the parameters aredetermined to be abnormal. In particular, the turbulence onset may beconsidered abnormal if there is a positive change in heart rateturbulence, and turbulence slope may be considered abnormal if theturbulance slope is less than 2.5 ms/RR-interval.

While methods described herein relate to detecting PVCs for use in HRTmonitoring, it is recognized that the methods described may beimplemented in any patient monitoring algorithm that requires reliablePVC detection. Apparatus and methods have been presented in theforegoing description with reference to specific embodiments. It isappreciated that various modifications to the referenced embodiments maybe made without departing from the scope of the disclosure as set forthin the following claims.

1. A method of distinguishing cardiac signals in a medical device,comprising: sensing cardiac signals; determining a plurality of R-wavesin response to the sensed cardiac signal; detecting a prematurecontraction being associated with a first R-wave of the plurality ofR-waves; determining whether a metric of the first R-wave is greaterthan a premature contraction threshold; and identifying the detectedpremature contraction as one of a premature atrial contraction or apremature ventricular contraction in response to determining whether ametric of the first R-wave is greater than a premature contractionthreshold.
 2. The method of claim 1, further comprising at least one ofgenerating an alert and storing the identifying of the detectedpremature contraction.
 3. The method of claim 1, further comprising:computing a morphology metric of an R-wave and a corresponding precedingmorphology metric of an immediately preceding R-wave; comparing one of adifference and a ratio of the R-wave morphology metric and the precedingR-wave morphology metric to the detection threshold; detecting apremature ventricular contraction in response to the comparison meetingthe detection threshold requirement; determining intervals betweenconsecutive R-waves; detecting a premature coupling interval; andcomputing the R-wave morphology metric and the preceding R-wavemorphology metric in response to detecting the premature couplinginterval.
 4. The method of claim 1, further comprising: computing amorphology metric of an R-wave and a corresponding preceding morphologymetric of an immediately preceding R-wave; comparing one of a differenceand a ratio of the R-wave morphology metric and the preceding R-wavemorphology metric to the detection threshold; and detecting a prematureventricular contraction in response to the comparison meeting thedetection threshold requirement, wherein computing the morphology metriccomprises: identifying a fiducial point along the R-wave; establishing afirst time interval ending at the fiducial point; establishing a secondtime interval beginning at the fiducial point; and computing a firstarea over the first time interval and a second area over the second timeinterval.
 5. The method of claim 1, wherein determining whether a metricof the R-wave is greater than a premature contraction thresholdcomprises: determining a maximum amplitude of the first R-wave; andcomparing the maximum amplitude to an amplitude threshold, wherein thedetected premature contraction is identified as a premature atrialcontraction in response to the maximum amplitude not being greater thanthe amplitude threshold and as a premature ventricular contraction inresponse to the maximum amplitude being greater than the amplitudethreshold.
 6. The method of claim 5, further comprising determining amaximum amplitude for a second R-wave of the plurality of R-waves,wherein the amplitude threshold corresponds to a percentage change ofthe determined maximum amplitude of the first R-wave from the maximumampliutude of the second R-wave.
 7. The method of claim 4, wherein thepercentage change is approximately 20 percent.
 8. The method of claim 5,further comprising: computing a morphology metric of an R-wave and acorresponding preceding morphology metric of an immediately precedingR-wave; comparing one of a difference and a ratio of the R-wavemorphology metric and the preceding R-wave morphology metric to thedetection threshold; detecting a premature ventricular contraction inresponse to the comparison meeting the detection threshold requirement;determining intervals between consecutive R-waves; detecting a prematurecoupling interval; and computing the R-wave morphology metric and thepreceding R-wave morphology metric in response to detecting thepremature coupling interval.
 9. The method of claim 5, furthercomprising: computing a morphology metric of an R-wave and acorresponding preceding morphology metric of an immediately precedingR-wave; comparing one of a difference and a ratio of the R-wavemorphology metric and the preceding R-wave morphology metric to thedetection threshold; and detecting a premature ventricular contractionin response to the comparison meeting the detection thresholdrequirement, wherein computing the morphology metric comprises:identifying a fiducial point along the R-wave; establishing a first timeinterval ending at the fiducial point; establishing a second timeinterval beginning at the fiducial point; and computing a first areaover the first time interval and a second area over the second timeinterval.
 10. The method of claim 1, further comprising computing ameasurement of the sensed ventricular R-waves in response to detectingthe premature ventricular contraction, wherein computing a measurementof the sensed ventricular R-waves comprises computing a heart rateturbulence measurement.
 11. A medical device for distinguishing cardiacsignals, comprising: a sensor sensing cardiac signals; and a processorconfigured to determine a plurality of R-waves in response to the sensedcardiac signal, detect a premature contraction being associated with afirst R-wave of the plurality of R-waves, determine whether a metric ofthe first R-wave is greater than a premature contraction threshold, andidentify the detected premature contraction as one of a premature atrialcontraction or a premature ventricular contraction in response todetermining whether a metric of the first R-wave is greater than apremature contraction threshold.
 12. The device of claim 11, furthercomprising: an alert module generating an alert in response to theidentifying of the detected premature contraction; and a storage devicestoring the identifying of the detected premature contraction.
 13. Thedevice of claim 11, wherein the processor is further configured tocompute a morphology metric of an R-wave and a corresponding precedingmorphology metric of an immediately preceding R-wave, compare one of adifference and a ratio of the R-wave morphology metric and the precedingR-wave morphology metric to the detection threshold, detect a prematureventricular contraction in response to the comparison meeting thedetection threshold requirement, determine intervals between consecutiveR-waves, detect a premature coupling interval, and compute the R-wavemorphology metric and the preceding R-wave morphology metric in responseto detecting the premature coupling interval.
 14. The device of claim11, wherein the processor is further configured to compute a morphologymetric of an R-wave and a corresponding preceding morphology metric ofan immediately preceding R-wave, compare one of a difference and a ratioof the R-wave morphology metric and the preceding R-wave morphologymetric to the detection threshold, and detect a premature ventricularcontraction in response to the comparison meeting the detectionthreshold requirement, wherein computing the morphology metriccomprises: identifying a fiducial point along the R-wave; establishing afirst time interval ending at the fiducial point; establishing a secondtime interval beginning at the fiducial point; and computing a firstarea over the first time interval and a second area over the second timeinterval.
 15. The device of claim 11, wherein the processor is furtherconfigured to determine a maximum amplitude of the first R-wave, andcompare the maximum amplitude to an amplitude threshold, wherein thedetected premature contraction is identified as a premature atrialcontraction in response to the maximum amplitude not being greater thanthe amplitude threshold and as a premature ventricular contraction inresponse to the maximum amplitude being greater than the amplitudethreshold.
 16. The device of claim 15, wherein the processor is furtherconfigured to determine a maximum amplitude for a second R-wave of theplurality of R-waves, wherein the amplitude threshold corresponds to apercentage change of the determined maximum amplitude of the firstR-wave from the maximum ampliutude of the second R-wave.
 17. The deviceof claim 16, wherein the percentage change is approximately 20 percent.18. The device of claim 15, wherein the processor is further configuredto compute a morphology metric of an R-wave and a correspondingpreceding morphology metric of an immediately preceding R-wave, compareone of a difference and a ratio of the R-wave morphology metric and thepreceding R-wave morphology metric to the detection threshold, detect apremature ventricular contraction in response to the comparison meetingthe detection threshold requirement, determine intervals betweenconsecutive R-waves, detect a premature coupling interval, and computethe R-wave morphology metric and the preceding R-wave morphology metricin response to detecting the premature coupling interval.
 19. The deviceof claim 15, wherein the processor is further configured to compute amorphology metric of an R-wave and a corresponding preceding morphologymetric of an immediately preceding R-wave, compare one of a differenceand a ratio of the R-wave morphology metric and the preceding R-wavemorphology metric to the detection threshold, and detect a prematureventricular contraction in response to the comparison meeting thedetection threshold requirement, wherein computing the morphology metriccomprises: identifying a fiducial point along the R-wave; establishing afirst time interval ending at the fiducial point; establishing a secondtime interval beginning at the fiducial point; and computing a firstarea over the first time interval and a second area over the second timeinterval.
 20. The device of claim 11, wherein the processor is furtherconfigured to compute a measurement of the sensed ventricular R-waves inresponse to detecting the premature ventricular contraction, whereincomputing a measurement of the sensed ventricular R-waves comprisescomputing a heart rate turbulence measurement.
 21. A non-transitorycomputer readable medium storing instructions which cause a medicaldevice to perform a method, the method comprising: sensing cardiacsignals; determining a plurality of R-waves in response to the sensedcardiac signal; detecting a premature contraction being associated witha first R-wave of the plurality of R-waves; determining whether a metricof the first R-wave is greater than a premature contraction threshold;and identifying the detected premature contraction as one of a prematureatrial contraction or a premature ventricular contraction in response todetermining whether a metric of the first R-wave is greater than apremature contraction threshold.